Passivation capping layer for ohmic contact in II-VI semiconductor light transducing device

ABSTRACT

A II-VI semiconductor device includes a stack of semiconductor layers. An ohmic contact is provided that electrically couples to the stack. The ohmic contact has an oxidation rate when exposed to an oxidizing substance. A passivation capping layer overlies the ohmic contact and has an oxidation rate that is less than the oxidation rate of the ohmic contact.

CROSS REFERENCE TO CO-PENDING APPLICATION

This is a Divisional of U.S. patent application Ser. No. 08/804,768,filed Feb. 24, 1997 now U.S. Pat. No. 5,767,534.

GOVERNMENT RIGHTS

The United States government has certain rights in this inventionpursuant to Contract No. DAAH 04-94-C-0049 awarded by the DefenseAdvanced Research Projects Agency and the Department of the Army/ArmyResearch Office.

BACKGROUND OF THE INVENTION

The present invention relates to II-VI semiconductor devices such aslaser diodes and light emitting diodes. More specifically, the presentinvention relates to a passivation capping layer for an ohmic contact toa II-VI semiconductor device.

Group II-VI compound semiconductor devices are known. Such devices areuseful in constructing light emitting or detecting devices, diodes andlaser diodes such as those described in U.S. Pat. No. 5,213,998, issuedMay 25, 1993; U.S. Pat. No. 5,248,631, issued Sep. 28, 1993; U.S. Pat.No. 5,274,269, issued Dec. 28, 1993; U.S. Pat. No. 5,291,507, issuedMar. 1, 1994; U.S. Pat. No. 5,319,219, issued Jun. 7, 1994; U.S. Pat.No. 5,395,791, issued Mar. 7, 1995; U.S. Pat. No. 5,396,103, issued Mar.7, 1995; U.S. Pat. No. 5,404,027, issued Apr. 4, 1995; U.S. Pat. No.5,363,395, issued Nov. 8, 1994; U.S. Pat. No. 5,515,393, issued May 7,1996; U.S. Pat. No. 5,420,446, issued May 30, 1995; U.S. Pat. No.5,423,943, issued Jun. 13, 1995; U.S. Pat. No. 5,538,918, issued Jul.23, 1996; and U.S. Pat. No. 5,513,199, issued Apr. 30, 1996.

Historically, diodes have generated red or infrared light. However,there are many applications where diodes that emit radiation in shorterwavelengths, for example, in the blue and green portions of the spectrum(i.e., at wavelengths between 590 nm and 430 nm) would be useful.Further, such short wavelength laser diodes would increase theperformance and capabilities of many existing systems that currently useinfrared and red laser diodes.

It is critical to obtain a good electrical contact to the device.Typically, II-VI semiconductor diodes have employed p-type ZnTe ohmiccontacts to provide an electrical conductance to p-type layers of aII-VI semiconductor diode. However, the interface between the ohmiccontact and the p-type layer is such that the valence band offset isapproximately 1 eV. This offset forms a barrier to hole injection. Agraded composition alloy layer has been used as a technique to removethis barrier and is described in U.S. Pat. No. 5,396,103. However, thereis a large lattice mismatch between the ohmic contact and the otherlayers of the device which gives rise to an increased density ofmicrostructural defects in the contact region. These defects may play asignificant role in degradation of the performance of the device.

SUMMARY OF THE INVENTION

The present invention includes a semiconductor device having a stack ofsemiconductor layers. A II-VI semiconductor ohmic contact electricallycouples to the device and includes beryllium (Be). The ohmic contact hasan oxidation rate when exposed to oxidizing environments. A passivationcapping layer overlies the ohmic contact and has an oxidation rate whichis less than the oxidation rate of the ohmic contact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram showing layers of a II-VIsemiconductor laser diode in accordance with the present invention.

FIG. 2 is a more detailed view of a p-type contact in the semiconductorlaser diode shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure of a laser diode 10 in accordance with the presentinvention is illustrated generally in FIG. 1. Laser diode 10 is a widebandgap II-VI device fabricated from heteroepitaxial layers grown bymolecular beam epitaxy (MBE) on a GaAs substrate. Laser diode 10 isfabricated on a GaAs substrate 12 and includes lower (first) and upper(second) ZnSSe light guiding layers 14 and 16, respectively, separatedby a CdZnSe quantum well active layer 18. The surfaces of light guidinglayers 14 and 16 opposite active layer 18 are bounded by lower (first)and upper (second) MgZnSSe cladding layers 20 and 22, respectively. Alower ZnSe:Cl buffer layer 24 is positioned on the surface of lowercladding layer 20 which is opposite light guiding layer 14. An upperBeTe:N/ZnSe ohmic contact 34 is positioned on the surface of uppercladding layer 22 which is opposite light guiding layer 16.

A GaAs buffer layer 28 separates substrate 12 from lower ZnSe:Cl bufferlayer 24 to assure high crystalline quality of the subsequently grownlayers. A p-type contact 34 is formed by ZnSe:N layer 26, ZnSe/BeTe:Ngrading layer 36, BeTe:N layer 38 and ZnTe:N capping layer 42. Electrode41 is provided for electrical contact to layer 42. Further, an electrode40 is provided to contact GaAs substrate 12 opposite lower buffer layer24. Layers 20 and 24 are all doped n-type with Cl (i.e., are of a firstconductivity type). Further, layers 22 and 26 are doped p-type with N(i.e., are of a second conductivity type). Active layer 18 is an undopedquantum well layer of CdZnSe or CdZnSSe semiconductor. Layers 12 through40 provide a “stack of semiconductor layers” in accordance with thepresent invention.

FIG. 2 is a more detailed version of p-type contact 34 in accordancewith the present invention deposited on p-type ZnSe layer 26. As shownin FIG. 2, digitally graded layers 36 comprise a plurality of layersstacked one upon the other. The p-type ZnTe capping layer 42 isdeposited upon p-type BeTe layer 38. In accordance with the invention,capping layer 42 has an oxidation rate which is less than the oxidationrate of the BeTe layer 38. Capping layer 42 has sufficient thickness andconsistency to completely cover layer 38. Therefore, capping layer 42protects layer 38 from exposure to air and other oxidizing agents andthereby prevents oxidation of layer 38. Those skilled in the art willrecognize that capping layer 42 may be of any appropriate material. ZnTeis selected because it is easily incorporated into the fabricationprocess of II-VI semiconductor devices. Other suitable capping layermaterials include CdSe, HgSe, and most metals.

One common low-resistance p-type ohmic contact to p-ZnSe based layersfor II-VI blue-green semiconductor laser diodes is p-ZnTe. The energyband lineup at the p-ZnTe/p-ZnSe interface is such that the valence bandoffset of approximately 1 eV forms a barrier to hole injection. Thecommonly used technique to remove the barrier is to introduce adigitally graded Zn(Se,Te) layer between the p-ZnSe and the p-ZnTe.However, the large lattice mismatch of the ZnTe/ZnSe layers gives riseto a high microstructural defect density in the contact regions, and mayplay a significant role in contact degradation. Since BeTe, with abandgap of 2.7 eV, is closely lattice-matched to ZnSe and GaAs, a BeTelayer with BeTe/ZnSe digitally graded layers appear an ideal candidatefor ohmic contact to p-type ZnSe based semiconductors.

A contact structure has been proposed which includes 200 Å p-BeTe and300-900 Å p-Be_(x)Zn_(1-x)Te_(y)Se_(1-y) linearly graded band gap ohmiccontact to p-ZnSe which should have superior electrical properties to ap-ZnTe_(x)Se_(1-x) linearly graded ohmic contact. Due to the smalllattice mismatch to p-ZnSe (˜0.7%) and GaAs, BeTe andBe_(x)Zn_(1-x)Te_(x)Se_(1-x) graded bandgap layers provide improvedohmic contacts to p-ZnSe based semiconductors. These contacts will allowfor entire II-VI laser structures to be grown pseudomorphically withGaAs substrates. Typically, following the deposition of the BeTe layer,the substrate is removed from the MBE chamber and is exposed to air orother oxidizing agents.

The present invention includes the discovery that BeTe is chemicallyactive in air and tends to react with oxygen to form an amorphous BeOlayer.

This oxidation of BeTe in air at room temperature has beenexperimentally observed using cross-sectional transmission electronmicroscopy (TEM).

This oxide layer is a potential problem during laser device processingand fabrication. The non-conductive oxide layer limits the efficiency ofthe laser diode and can also shorten the lifetime of the laser duringroom temperature continuous wave operation. It is therefore necessary toprevent BeTe surface oxidation in air. The present invention addressesthis issue with an electrically conductive capping layer subsequentlydeposited onto the BeTe surface before the BeTe surface is exposed toair. In one embodiment, the capping layer is a ZnTe layer.

The advantages of the present invention have been observedexperimentally. In one experiment, two contacts consisting of 300 Åp-BeTe and 300 Å ZnSe/BeTe digitally graded layers were grown by MBEonto p-ZnSe. A 50 Å capping layer of p-ZnTe was subsequently depositedonto one of the two 300 Å p-BeTe finished surface. The two wafers werethen exposed to air for device processing. The two contact structureswere characterized by cross-sectional transmission electron microscopy(TEM).

A thin amorphous layer at the top of the uncapped BeTe was observed,while no amorphous layer was observed between the ZnTe capping layer andthe BeTe layer. Thus, it has been discovered that BeTe is subject tooxidation even at room temperature in air. On the other hand, a ZnTecapping layer passivated the BeTe surface and no oxide layer wasobserved. Such BeTe contacts with a ZnTe capping layer have had contactlife times under accelerated testing at a current density of 2 kA/cm² inexcess of 600 hr. This compares to ZnTe based contacts which haveoperated for only 20 hr at 1 kA/cm².

The invention may employ other materials with relatively high resistanceto oxidation that are preferably electrical conductors. For example,p-CdSe, p-CdTe, p-HgS, p-HgSe, etc. can be used as capping layers forBeTe. In addition, ZnSe can be used as a capping layer if the layer isthin enough (up to about 100 Å) for electron tunnelling to occurtherethrough. Other materials which are resistive in their bulk form maybe selected if they are sufficiently thin to provide electricalconduction. Further, in-situ metal contacts with high work functions,such as Pt, Pd, Ir, Rh, Ni, Co and Au, can be directly deposited ontothe BeTe surface without exposure to air to passivate the BeTe surfacesduring fabrication. Even lower work function metals are acceptable, forexample, Al, Ti, Zn and Cd. It is within the scope of the invention tocap any layer in a II-VI semiconductor which tends to oxidize. Suchlayers may consist of BE-containing compounds which have been dopedp-type or n-type. In the case of an n-doped semiconductor layer, thecapping layer would also be doped n-type to achieve good electricalconduction.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. As used herein, “stack of II-VIsemiconductor layers” includes any layer or grouping of layers includinglight producing devices, the laser diodes set forth herein or otherconfigurations such as diodes, devices with n-type up, etc. Theinvention is well adapted for any situation in which an easily oxidizedlayer of a II-VI device must be exposed to oxidizing agent.Semiconductor devices of the invention are useful as lasers, lightemitting diodes, sensors, etc. Devices using semiconductors of theinvention include optical communication devices, optical storage,optical read-write devices, etc.

What is claimed is:
 1. A method of making a TT-VT semiconductor device,comprising: placing a substrate in an MBE chamber; depositing a stack ofsemiconductor layers through MBE deposition while the substrate is inthe chamber; depositing onto the stack of semiconductor layers an ohmiccontact of II-VI semiconductor containing BeTe through MBE depositionwhile the substrate is in the chamber, the ohmic contact electricallycoupling to the stack and deposited in a vacuum without exposure to air,wherein the ohmic contact is of a material prone to oxidation;depositing an electrically conductive passivation capping layeroverlying the ohmic contact whereby an electrical current can be appliedto the stack of semiconductor layers through the electrically conductivepassivation capping layer and the ohmic contact to thereby energize thedevice, the depositing performed in vaccum without exposure of the ohmiccontact to air, the capping layer having an oxidation rate which is lessthan an oxidation rate of the ohmic contact when exposed to air thepassivation capping layer including materials selected form the groupconsisting of ZnTe, CdSe, CdTe, HgS, HgSe, ZnSe and a metal; and whereinduring the step of depositing the ohmic contact, during step ofdepositing the passivation capping layer, and during a period betweenthe steps, the ohmic contact is not exposed to air.
 2. The method ofclaim 1 wherein the step of depositing the stack of semiconductor layerscomprises depositing a stack of semiconductive layers to form II-VIlight emitting diode.
 3. The method of claim 1 wherein the step ofdepositing the stack of semiconductor layers comprises depositing astack of semiconductive layers to form II-VI laser diode.
 4. The methodof claim 1 including doping the ohmic contact p-type.
 5. The method ofclaim 1 including doping the ohmic contact n-type.
 6. The method ofclaim 1 including doping the passivation capping layer p-type.
 7. Themethod of claim 1 including doping the passivation capping layer n-type.8. The method of claim 1 wherein the metal comprises Pt.
 9. The methodof claim 1 wherein the metal comprises Pd.
 10. The method of claim 1wherein the metal comprises Ir.
 11. The method of claim 1 wherein themetal comprises Rh.
 12. The method of claim 1 wherein the metalcomprises Ni.
 13. The method of claim 1 wherein the metal comprises Co.14. The method of claim 1 wherein the metal comprises Au.
 15. The methodof claim 1 wherein the metal comprises Al.
 16. The method of claim 1wherein the metal comprises Ti.
 17. The method of claim 1 wherein themetal comprises Zn.
 18. The method of claim 1 wherein the metalcomprises Cd.
 19. The method of claim 1 including removing the substratefrom the chamber subsequent to the step of depositing the passivationcapping layer and thereby exposing the capping layer to air.